1. Field of the Invention
The present invention relates to a semiconductor device and method of forming the same, and more particularly to a semiconductor device having a silicide thin film and method of forming the same.
2. Description of the Related Art
Silicon generally has the properties of semiconductor, but acts as a conductor when impurities are implanted into it, as is the case when used in a semiconductor device. In addition, silicon and metal can be easily transformed into a metal silicide having high conductivity. Accordingly, in a highly-integrated semiconductor device in which resistance increases as the width of a conductive line such as a gate line and contact sizes are reduced, metal silicide is often used to form a portion of a contact interface or a signal line such as the gate line to enhance the conductivity and the performance of the device.
Also, as semiconductor devices are scaled down, the junction depths of source/drain regions are also reduced. To reduce the contact resistance of the source/drain regions, metal silicide can again be used. At this time, a layer of metal silicide is generally formed to a thickness of several hundred angstroms (Å). However, for example, when the metal silicide layer having a thickness of 300 Å is formed on the source/drain regions having a thin junction depth of 1000 Å, the metal silicide layer may be directly connected to the substrate beyond the source/drain regions. Consequently, various problems, such as a junction spiking phenomenon generating a leakage current, can occur. This junction spiking phenomenon is similar to the spiking problem in which the signal current leaks into the substrate when aluminum contact plugs are connected to the source/drain regions.
Cobalt or titanium (Ti) metals, having a low contact interface resistance, are commonly used to form the metal silicide layer. With cobalt, however, the probability of encountering the junction spiking phenomenon is increased. The cobalt silicide layer is usually formed by coating a cobalt layer on the exposed surface of a substrate through sputtering. Sputtering is followed by a two-step heat treatment, in which a first step takes place at temperatures of 500 to 600° C. and a second step takes place at more than 750° C., or a one-step heat treatment that takes place at a high temperature of more than 750° C. The heat treatment silicifies the cobalt layer. After the heat treatment, non-reacted portions of the cobalt layer are removed in a self-aligned manner by wet etching. Using this technique, it is difficult to control a speed of forming the cobalt silicide layer to form a thin layer. When the cobalt silicide layer is formed, the polysilicon layer and the cobalt layer are usually combined in a ratio of 360 Å to 100 Å. It is further difficult to coat the cobalt layer uniformly at a thickness of less than 80 Å through sputtering and to re-form it repeatedly. The reliability of fabrication process is therefore deteriorated. Thus, forming the cobalt silicide layer of less than 300 Å, as well as preventing the junction spiking phenomenon in the source/drain regions having the thin conjunction depth, is difficult.
When the signal current leaks into the substrate without being transmitted through channels, the consumption of the signal current is increased and the operation speed of transistors slows. Worst yet, the transistors may not operate properly. Particularly, in a low power SRAM device, it is necessary to prevent the generation of leakage current to obtain reliable operation of the device.
To prevent the spiking phenomenon that results from using the cobalt silicide layer, a titanium silicide layer (TiSi2) in the source/drain regions can be formed using titanium to reduce the spiking phenomenon. In this case, however, because titanium exhibits an amount of resistance that depends on the line width, the resistance is abruptly increased in most semiconductor devices having a line width of less than 0.2 μm. Also, the resistance characteristics of the titanium may be degraded following heat treatment.